Recently, a semiconductor element made of a nitride semiconductor such as gallium nitride (GaN) has been researched and developed actively. A semiconductor element composed of nitride semiconductors including aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN) or mixed crystal thereof absorbs light in a wide wavelength region from ultraviolet or blue to infrared by varying its film composition. Furthermore, the light absorption coefficient of the nitride semiconductor is more than one-hundred times higher than that of silicon that is a material used widely as a semiconductor material for a solar cell. For this reason, as its application, a solar cell using a columnar nitride semiconductor was proposed (for example, Patent Literature 1).
FIG. 8 shows a cross-sectional view of a solar cell made of a columnar nitride semiconductor disclosed in Patent Literature 1. As shown in FIG. 8, the columnar nitride semiconductor disclosed in Patent Literature 1 includes a first electrode 2a provided on the first surface of a substrate 6a, a semiconductor region 8a formed on the second surface of the substrate 6a, a plurality of first nano-columns 12a made of InGaN and formed on the semiconductor region 8a, a plurality of second nano-columns 14a made of InGaN having difference polar character formed on the first nano-columns 12a, and a second electrode 24a provided on the second nano-columns 14a. 
The solar cell disclosed in Patent Literature 1 improves conversion efficiency from light energy to electric energy by using the columnar nitride semiconductor, compared to a solar cell using nitride semiconductor composed of stacked films.
In order to improve the conversion efficiency of the solar cell disclosed in Patent Literature 1, a nitride semiconductor film having significantly low defects in the crystal has to be prepared so as to suppress the carrier recombination due to non-radiative transition derived from lattice defects or threading dislocations. For this reason, a single-crystalline substrate such as a sapphire substrate is used, but such a single-crystalline substrate is very expensive.
In order to improve the conversion efficiency from light energy to electric energy, it is necessary to suppress temperature increase of the solar cell. This requires the substrate used for forming the nitride semiconductor to have high thermal-conductivity. However, a thermal conductivity of a single-crystalline substrate such as a sapphire substrate is low.
In order to solve the above-mentioned problems, Patent Literature 2 discloses a method for fabricating a poly-crystalline nitride semiconductor film by a pulse sputtering method on a graphite film, which is available at low cost. A graphite film has high thermal-conductivity. In this method, the graphite film is used as a substrate.